UPDATE: Audio files of the oral argument in this appeal are available on the Web site of the U.S. Court of Appeals for the Third Circuit, here and here.

If the FBI wants to know where an individual is, and if the Department of Justice prevails in a caserescheduled for argument tomorrow in snowy Philadelphia, the FBI (or other law enforcement authorities) will be able to obtain that individual's cell site data from the individual's cellular carrier on a showing of "reasonable grounds" to believe that the data is "relevant and material to an ongoing investigation." This is an issue that may have important implications for law enforcement, with significant impact on wireless carrier operations (and costs), and dramatic implications for privacy in the wireless world. In In re Application of the United States of America, No. 08-4227 (Third Cir.), attorneys for the Electronic Frontier Foundation, arguing as amicus curiae, will urge the appeals court to uphold a lower court ruling that applications for cell site data should be supported by a showing that satisfies the higher probable cause standard under the Fourth Amendment.

The many millions of cell phone users in the U.S. may not realize that their phones are giving away their location about every 7 seconds when they receive and return a “ping” from the nearest cell phone tower. A single “ping” indicates that the cell phone is within about a three-mile radius from the tower, but in metropolitan areas in which cell phone tower signals overlap, triangulation of these pings from multiple towers can narrow that down to about fifty feet.

Not only are subscribers likely unaware that this data is being generated in the first place, they are also unlikely to be aware when their data is actually obtained by law enforcement authorities. Unlike a law enforcement search of a home or office where police officers show up in person and serve a warrant on the occupant, a warrant or other order seeking the turnover of cellular subscriber data is served on the carrier, which has no obligation to notify the subscriber that such a demand has been made. On the contrary, such orders and requests often include a gag order prohibiting disclosure and the proceedings by which they were granted are often sealed from public access.

The usefulness of this data to law enforcement for surveillance and investigative purposes is obvious. It is so useful that law enforcement authorities make requests for this type of information so frequently that cellular carriers maintain staffs that are devoted to the task of responding to these requests. Carriers are concerned about the significant cost associated with the providing this information to authorities, even though they are permitted by law to charge authorities for access to it. In response to a public report that set the number of requests to a particular major carrier in the millions, the carrier responded that such requests numbered in the "thousands” in the previous 13 months. This particular carrier revealed that it is now making a Web-based application available to law enforcement authorities to facilitate their access.

The importance of locational data, to subscribers and law enforcement authorities, can only increase with the proliferation of GPS-enabled smartphones. While locational information generated from cell site data can be revealing, GPS-enabled applications have the potential to reveal even more sensitive information about subscribers. A GPS-enabled smartphone can be located much more precisely than an ordinary cell phone. And, consider the privacy implications of smartphone apps that might be used to locate, say, the nearest XXX-rated video store, or the nearest all-night pharmacy that sells the morning-after pill, or the nearest AA meeting.

If the appeals court agrees that prosecutors must meet the higher, "probable cause” standard for obtaining subscriber locational data, then carriers will be processing fewer requests for cell site data. That should mean that carriers will reduce their cost of compliance and subscribers can feel more secure in using cell phone functionality that relies on a stream of possibly revealing locational data.